Method of anodizing steel

ABSTRACT

A method of anodizing non-stainless steel, wherein a non-stainless steel object is connected to a positive terminal of a power supply, a counter electrode or vessel is connected to a negative terminal of the power supply, the non-stainless steel object and counter electrode are placed into a solution of NaOH, and a voltage is applied across the terminals to anodize the non-stainless steel object by forming an adherent blue-black or semi-adherent dichroic, colored oxide coating thereon.

The present application is a CIP application of U.S. application Ser. No. 11/624,137. Therefore, the present application should be granted the priority date of Jan. 17, 2007, the filing date of the corresponding patent application Ser. No. 11/624,137.

BACKGROUND OF THE INVENTION

The present invention relates to a method of anodizing iron or steel, in particular non-stainless steel, i.e. carbon steel that contains less than 1% chromium.

Bare steels rust when exposed to fresh water, salt water, or high condensing humidity. The corrosion products on such steel after atmospheric exposure are flaky and non-adherent rust. The prior art methods of providing a barrier layer between the steel and the environment have proven to be unsatisfactory for many different reasons. It is therefore an object of the present application to provide a method of anodizing steel to form an adherent oxide coating on the steel.

BRIEF DESCRIPTION OF THE DRAWINGS

This object, and other objects and advantages of the present application, will appear more clearly from the following specification in conjunction with the accompanying drawings, in which:

FIG. 1 schematically indicates a test vessel for anodizing a steel object;

FIG. 2 is a graph showing the oxide film growth on steel objects for five minutes at different conditions;

FIG. 3 a micrograph showing the cross-section of the anodized film on steel;

FIG. 4 is the graph of the glancing angle X-ray diffraction spectra of the oxide films;

FIG. 5 is a graph showing the fractured cross-section of the film; and

FIG. 6 is a graph showing the improved corrosion resistance of the films.

SUMMARY OF THE INVENTION

The method of anodizing steel pursuant to the present application includes the steps of connecting a steel object to a positive terminal of a power supply, connecting a counter electrode to a negative terminal of the power supply, placing the steel object and counter electrode into a 50% by weight solution of NaOH, and applying a voltage across the terminals to anodize the steel object, wherein applying the voltage results in the formation of an adherent blue-black or a colored semi-adherent dichroic oxide coating on the steel object. “Dichroic” refers to a surface that reflects different colors when viewed at different angles.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring now to the drawings in detail, FIG. 1 schematically indicates how applicants' method of anodizing steel can be carried out. In the illustrated embodiment, a 50% solution of NaOH is provided in an appropriate steel vessel 10. A non-stainless steel object 12, which is preferably first cleaned and then rinsed with deionized water and then rinsed with methanol, is connected to the positive terminal 14 of a power supply 16, and the steel vessel 10, (which acts as the counter electrode) is connected to the negative terminal 20 of the power supply 16. The power supply is then turned on, and a voltage is supplied across the positive and negative terminals 14, 20, thereby anodizing the steel object 12. In particular, an adherent oxide coating or protective oxide film is formed on the steel object. This oxide coating is essentially a disordered or nanometer-size crystalline magnetite (Fe₃O₄). The solution is preferably rapidly stirred during the anodization process to obtain a uniform surface, and is also heated, as will be discussed subsequently.

Although the counter electrode/vessel 10 can also be made of steel, it could also be made of any other material that can conduct electricity and that does not corrode in NaOH, such as, by way of example only, platinum or nickel.

By way of example only, the electrodes formed by the steel object 12 and the counter electrode/vessel 10 can be spaced 7 cm apart for a two-electrode system using the voltages reported herein. It should furthermore be noted that a three-electrode system could also be used, and the required voltages would change accordingly.

The presently preferred concentrations for the electrolyte solutions are 50% by weight NaOH. The 50% NaOH solution can be prepared by adding 760 g of deionized water to 760 g of NaOH to make one L of solution. Tests resulting in the data of the graphs of FIGS. 2-6 were conducted using 50% NaOH solutions.

Although it was indicated above that the solution could be heated, the temperature of the solution during anodization can be anywhere from 30° C. to the boiling point of the solution.

FIG. 2 is a graph showing the type of oxide film which will grow in five minutes at a given temperature and applied potential. For the regions labeled dichroic, the oxide color depends on the viewing angle and the thickness of the films. The oxide colors can very from blue to violet, to green, to gold. The black films from only in a very narrow region of temperature and potential. As the temperature increases, the films become thicker but less adherent.

FIG. 3 shows the scanning electron micrographs of the cross-sections of the two different steel samples anodized under different conditions. The anodized oxide is of uniform thickness. The FIG. 3 b shows a thick oxide which is 15 pm thick and is beginning to crack

FIG. 4 shows the x-ray diffraction spectra for oxides grown at different temperatures. The oxides all exhibit the magnetite Fe₃O₄ peaks which are labeled “mag”. The broad width of the peaks indicates the magnetite crystal size is in the range of nanometers.

FIG. 5 shows the fractured cross-section of the anodized films. The oxide is composed of porous channels.

FIG. 6 shows the corrosion resistance of the anodized film in oxygenated saltwater environments. The filling of the pores with WD40 oil greatly increases the corrosion resistance, decreasing the corrosion rate.

For a uniform, thick, blue-black adherent anodic oxide, the preferred voltages to be applied across the terminals of the power supply range from 1.5 to 3.0V, and the temperatures range from 30-115° C., for a NaOH solution. In addition, the voltage can be applied for from a few seconds to many hours, depending upon the desired thickness of the oxide coating that is to be formed.

Potential applications for applicants' method of anodizing steel include corrosion protection, pre-weathering of weathering steels, conversion coating to improve the adherence of organic coatings, such as paints, internal protection of, for example, boiler tubes, and architectural colored highlights.

The specification incorporates by reference the disclosure of the corresponding U.S. application Ser. No. 11/624,137, which was filed on Jan. 17, 2007.

The present invention is, of course, in no way restricted to the specific disclosure of the specification and drawings, but also encompasses any modifications within the scope of the appended claims. 

1. An apparatus, comprising: an anodized non-stainless steel object, wherein said object is provided with an adherent blue-black or colored semi-adherent dichroic anodic oxide film directly on a steel surface of said object, and wherein the oxide film has a thickness of from 0.04 to 15 microns.
 2. An apparatus according to claim 1, wherein the oxide film is essentially a nano-crystalline magnetite (Fe₃O₄).
 3. A method of anodizing non-stainless steel, including the steps of: a) connecting a non-stainless steel object to a positive terminal of a power supply; b) connecting a counter electrode to a negative terminal of the power supply or to a vessel; c) placing the non-stainless steel object and counter electrode into a solution of NaOH in the vessel; and d) applying a voltage across the terminals to anodize the non-stainless steel object by growing an adherent blue-black or colored semi-adherent dichroic oxide film directly on a steel surface of the object.
 4. A method according to claim 3, wherein said solution is stirred and/or heated during said step of applying a voltage.
 5. A method according to claim 3, wherein said oxide film is essentially a nano-crystalline magnetite (Fe₃O₄).
 6. A method according to claim 3, wherein said counter electrode is steel, platinum, nickel, or any other material that can conduct electricity and does not corrode in NaOH.
 7. A method according to claim 3, wherein said solution is at a temperature in the range of from room temperature to the boiling point of the solution.
 8. A method according to claim 3, wherein said solution is NaOH at a concentration of about 50% by weight.
 9. A method according to claim 3, wherein the voltage applied across the terminals is from 1.7 to 2.2V in order to grow a black adherent oxide.
 10. A method according to claim 9, wherein said voltage is applied for from a few seconds to several hours, as a function of a desired oxide film thickness to be formed thereby.
 11. A method according to claim 9, wherein the temperature of said solution is from 40-80° C.
 12. A method according to claim 3, wherein the voltage applied across the terminals is greater than 1.5V and the temperature and the time of application are such as to obtain a specified color of dichroic oxide film.
 13. A method according to claim 3, wherein said film has a thickness of from 0.04 to 15 microns. 